The present invention relates to a diagnostic method and system for at least one ball bearing, in particular for at least one angular-contact ball bearing for bearing a rapidly rotating spindle, and also to a use of such a diagnostic system for diagnosing at least one ball bearing, in particular at least one angular-contact ball bearing, for bearing a spindle of a spindle system, in particular of a machine tool.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Ball bearings of a type involved here include deep groove ball bearings and, in particular, angular-contact ball bearings. The latter are designed to absorb both axial and radial forces with reference to their rotational axis. They are mostly installed and prestressed in pairs in order to support a shaft. Such angular-contact ball bearings can be of single row or as double row design. Furthermore, such angular-contact ball bearings have a bearing inner race and a bearing outer race with a multiplicity of interposed bearing balls guided in a bearing cage. The ability to be able to absorb both axial and radial forces is achieved by inclining the rolling paths of the bearing balls. A measure of the inclination is the so-called pressure angle α. Typical values are at 15°, 18°, 25° or 45°. Other pressure angles are also possible.
By comparison therewith, deep groove ball bearings have a pressure angle of 0°. They are designed, in particular, to absorb radial forces.
Spindle systems with rapidly rotating spindles or spindle shafts are typically supported in two angular-contact ball bearings. This makes possible axial and radial guidance of the spindle shaft. Such rapidly rotating spindle systems serve to hold, for example, a work piece, a tool, a milling head or a drill. Such spindle systems are frequently part of a machine tool.
In such spindle systems, continuous monitoring of the angular-contact ball bearings is frequently necessary or even prescribed, in order to detect, preferably to predict, a failure or an indicated failure. However, this has so far not been possible with a sufficiently high reliability, since there are no generally valid, informative limit values relating to the probability of failure of rapidly rotating angular-contact ball bearings. It Although limit values suitable for specific machines depending on process, in particular machine tools, can be determined for spindle systems fabricated in large piece numbers. However, these limit values can be transferred only conditionally to similar machine tools and similar processes. Because of the high outlay on metrology and of the high costs associated therewith, no automated analysis of angular-contact ball bearings is currently undertaken.
In order to diagnose rapidly rotating angular-contact ball bearings, it is also known to apply acceleration sensors in the vicinity of angular-contact ball bearings and to evaluate the acquired acceleration signals. Thus, for example, the total value can be formed together with vibration data corresponding to the acceleration signals, in order to signal a variation in the bearing state given a variation in the total value. However, it is mostly impossible here to assign the vibration data clearly to the respective affected machine component or to a present cause.
As an alternative, methods are known that perform a frequency selective evaluation of the acquired vibration data. By contrast with the evaluation of the total signals, it is possible when conducting frequency selective evaluation to demonstrate individual causes of fault, and to assign the machine elements affected. In this case, the time signal is decomposed by means of a Fourier transformation into the corresponding sinusoidal vibration components. The respective amplitudes or levels of the associated frequencies are displayed in the spectrum. However, spectra cannot be used for early detection of rolling bearing defects, because the extremely low energy defect components or frequency components are mostly covered by high energy machine vibrations.
In another alternative method, the spectrum of the envelope can be determined from the acceleration signal. This method can used to separate the periodic force pulses that arise when a defect in a bearing is rolled over and are superposed on one another in the machine vibrations. In this process, the time signal modulated by defect pulses is demodulated, that is to say the envelope is separated and subjected to frequency analysis. Given suitably filtered time signals, it is only exclusively information relating to periodic rolled-over defects that is displayed in the spectrum.
Stochastic signal components are suppressed, with the result that even small defects can be displayed independently of the machine vibrations, which are substantially higher in energy. This method has a comparatively good signal-to-noise ratio, and it is, in addition, relatively insensitive to rotational speed fluctuations. Instances of early damage can mostly be detected easily with this method. In addition, it is possible to analyze the damage growth with time in the rolling bearings.
Disadvantageously, these measurements and the required outlay on parameterization for a respective angular-contact ball bearing and for the affected machine are complex.
It would therefore be desirable and advantageous to provide an improved diagnostic method and system for at least one ball bearing, in particular for at least one angular-contact ball bearing to obviate prior art shortcomings.